In recent years, as heat-generating electronic components such as ICs and MPUs have been smaller and thinner and become higher functional, an amount of heat generated by electronic devices equipped with electronic components has increased, and the development of an efficient heat dissipation method has been still expected. The heat dissipation of an electronic device has been done by attaching a heat sink to a substrate equipped with heat-generating electronic components, or by directly attaching a substrate to a metal chassis of the electronic device when a space for installing the heat sink cannot be ensured. In this case, the heat sink has been attached via a heat radiating member such as a sheet formed by filling inorganic powder having good electric insulation properties and thermal conductivity, for example inorganic powder such as boron nitride powder, aluminum nitride powder and alumina powder, into a silicone rubber, or a flexible sheet having an Asker C hardness of 25 or less (Patent Document 1).
The quality of heat dissipation properties in a resin composition after molding is greatly influenced by thermal conductivity of the resin composition after the molding and adhesiveness to the adherend (shape followability), and also affected by the presence or absence of voids (air layers) included in the resin composition. The thermal conductivity is maintained by incorporating the inorganic powder in a higher concentration, but when the inorganic powder are highly filled into a resin or the like, the fluidity of the resin composition before molding is greatly deteriorated, so that the moldability is impaired and adhesion is remarkably deteriorated. On the other hand, as the viscosity of the resin composition before molding increases, the voids included therein are difficult to be eliminated, so that the thermal conductivity also decreases. Based on this, to maintain the filling rate of the inorganic powder to a certain extent so that both of the fluidity and the higher thermal conductivity of the resin composition before molding are achieved, while not to significantly impair the moldability and adhesiveness, the use of spherical alumina powder and alkoxysilane compounds has been proposed (Patent Document 2).
Further, the higher filling of the inorganic powder not only impairs the fluidity of the resin composition before the molding, but also significantly reduces heat resistance reliability of mechanical properties of the molded resin composition, such as an increase in permanent compression set and a decrease in tensile strength of the resin composition after the molding. As a method for improving the heat resistance reliability of the mechanical properties, it has been proposed to treat the surfaces of the inorganic powder with an alkoxysilane compound(s) having a long chain alkyl group(s) (Patent Document 3).
In the surface treatment of spherical alumina powder using a trialkoxysilane compound having three functional groups, not all of the three functional groups of the trialkoxysilane compound react with the surfaces of the inorganic powder, leaving some unreacted functional groups. The remaining unreacted functional groups would allow disordered progress of polymerization of the trialkoxysilane compounds over time due to effects of temperature and humidity. There has been a problem that if the polymerization with the reaction between the trialkoxysilanes could not be controlled, viscosity of the resin composition before the molding process was increased to impair fluidity as compared with the untreated inorganic powder due to effects of the polymerized silane compounds and the spherical alumina.